Liquid crystal molecules having polymerizable functional groups have both characteristics as a polymerizable monomer and characteristics as a liquid crystal. Accordingly, if they are polymerized in a state where they are aligned, it is possible to obtain a polymer with fixed alignment, i.e. an optical anisotropic material. The optical anisotropic material thus obtained has an optical anisotropy based on the refractive index anisotropy derived from a mesogen skeleton and is applied to e.g. an optical element such as a diffraction element or a phase plate by the use of such a characteristic.
A phase plate using a polymer liquid crystal is known to have low temperature dependence of phase difference (or retardation) as compared with a phase plate using a low molecular weight liquid crystal.
As such a phase plate, for example, Patent Document 1 discloses “a phase plate comprising a quarter wave plate to create a quarter wave phase shift of birefringent light and a half wave plate to create a half wave phase shift of birefringent light in a state where their optical axes intersect with each other”, and discloses that the phase plate is prepared by laminating a quarter wave plate and a half wave plate obtained by stretching a polymer film (particularly polycarbonate). However, for the phase plate disclosed in Patent Document 1, a plurality of wave plates have to be laminated, and complicated production process is required.
Further, Patent Document 2 discloses “a phase difference film having a liquid crystal layer containing a compound having two or more kinds of mesogen groups and a rod-like liquid crystal compound, wherein the rod-like liquid crystal compound is homogenously aligned, and at least one kind of the mesogen group of the compound containing mesogen groups is aligned in a direction of from 45 to 90 degree with the direction of the optical axis of the rod-like liquid crystal compound in a plane of a film”, and discloses that a phase plate utilizing wavelength dispersion of a phase difference can be produced by such a film. However, since two different materials of the compound containing two or more kinds of mesogen groups and the rod-like liquid crystal compound are required, the degree of freedom in design is restricted, such that various combinations must be considered to meet desired properties, and materials appropriate for the combination must be selected and obtained.
Further, Patent Document 3 discloses “a four-membered cyclic compound represented by the following formula (I):
wherein each of X1 and X2 which are independent of each other, is an oxygen atom, a sulfur atom or the like, each of Y1 and Y2 which are independent of each other, is a single bond, an oxygen atom, or the like, each of B1 and B2 which are independent of each other, is a C1-20 aliphatic group which may have a substituent, or the like, each of two benzene rings directly bonded to the cyclobutane ring may have a substituent on the ring, and each of A1 and A2 which are independent of each other, is a group represented by the following formula (II):
wherein each of Ar1, Ar2 and Ar3 which are independent of each other, is a C5-14 cyclic group, or the like, each of L1 and L2 which are independent of each other, is a single bond or a bivalent connecting group, and p is an integer of from 0 to 2, provided that when p is 2, two Ar2's and two L2's may be the same or different”, and discloses that a phase plate which controls wavelength dispersion of a phase difference can be produced by such a compound.
Further, Patent Document 4 which discloses an invention focusing on a tolan liquid crystal to be used for a liquid crystal element for high resolution polymer-dispersed type liquid crystal display, discloses, as a polymerizable liquid crystal compound, a liquid crystal compound containing —C≡C— as a connecting group. As one of many examples, a polymerizable liquid crystal compound represented by the following formula (III) having a —C≡C— connecting group between a naphthalene-1,4-diyl group and a 1,4-phenylene group, is disclosed in paragraph number 0083:
wherein R1 is a hydrogen atom, an alkyl group, a phenyl group or a halogen.
Patent Document 1: JP-A-10-68816
Patent Document 2: JP-A-2002-267838
Patent Document 3: JP-A-2003-160540
Patent Document 4: JP-A-7-082183